There is well known a vehicle provided with a mechanically operated step-variable transmission disposed between a drive power source and drive wheels. JP-2006-9942A discloses an example of this type of vehicle. This vehicle is a hybrid vehicle the drive power source of which includes an engine, and first and second electric motors which are operatively connected in a power transmittable manner to planetary gear sets functioning as a differential mechanism, and various rotary elements of the planetary gear sets. JP-2006-9942A also discloses a technique for calculating output torques of the first and second electric motors on the basis of their electric current values, estimating an input torque of the step-variable transmission on the basis of the calculated output torques of the first and second electric motors, and controlling transient hydraulic pressures to be applied to coupling devices of the step-variable transmission to implement its shifting actions, on the basis of the estimated input torque.
By the way, the transient hydraulic pressures to be applied to the coupling devices in the process of the shifting actions of the step-variable transmission are controlled on the basis of the input torque of the step-variable transmission, in a vehicle the drive power source of which consists solely of an engine, as well as in the hybrid vehicle the drive power source of which includes the electric motors as disclosed in JP-2006-9942A. A power-on shift-down action of the step-variable transmission is required to have a high degree of control response, so that the power-on shift-down action is desirably implemented immediately after a moment of determination to implement the power-on shift-down action. If a shift-down command to implement the power-on shift-down action is generated immediately after the moment of determination to implement the power-on shift-down action, however, the engine torque (engine power as well) has not been fully raised at a point of time of initiation of an inertia phase of the power-on shift-down action of the step-variable transmission, so that the engine torque and the engine power may increase at a high rate after the initiation of the inertia phase. Accordingly, there is a risk of generation of a considerable engaging shock of a coupling device of the step-variable transmission in the process of an engaging action of the coupling device, due to a high rate of rise of a rotating speed of an input shaft of the step-variable transmission, namely, a racing of the input shaft, which is caused by surplus torque and power of the engine. In the hybrid vehicle in which an electric motor is operatively connected in a power transmittable manner to the input shaft of the step-variable transmission, for example, the engaging shock of the coupling device can be reduced by the electric motor which absorbs the surplus torque and power of the engine. However, a regenerative operation of the electric motor increases a load on a battery, and it is difficult for the electric motor to absorb the surplus torque and power of the engine, when the regenerative operation of the electric motor is limited, so that the engaging shock of the coupling device is undesirably increased in the process of the shift-down action of the step-variable transmission.